To bring a vessel or the like, which is an object to be vacuumized, into a vacuum state, an oil-sealed rotary vacuum pump is directly connected to the vessel or the like. By so doing, an ideal exhaust system is formed, and the vacuum state can be achieved most easily and inexpensively. In this case, however, if the oil-sealed rotary vacuum pump is stopped, with the vacuum vessel being maintained in a vacuum, the oil-sealed rotary vacuum pump is also in a vacuum state. Thus, the lubricating oil of the oil-sealed rotary vacuum pump flows into a rotor chamber of the pump. When the rotor chamber is filled with the oil, the oil is pushed up from a pump inlet toward the upstream side.
If backflow of the oil occurs, as mentioned above, such drawbacks as the following occur: Vacuum piping and the vacuum vessel are contaminated and, in subsequent vacuum evacuation, the atmosphere of the vacuum vessel is contaminated with hydrocarbons from a vapor of the oil. As a result, ultimate pressure is increased, and carbon is deposited on the object to be vacuumized.
To avoid such drawbacks, an exhaust system as shown in FIG. 1 is generally used. That is, before an oil-sealed rotary vacuum pump 100 is stopped, a shut-off valve (V1) is closed to isolate and block a vacuum vessel 101 and the oil-sealed rotary vacuum pump 100 from each other. Then, an air introduction valve (V2) is opened to restore a rotor chamber of the oil-sealed rotary vacuum pump 100 to atmospheric pressure, whereafter the oil-sealed rotary vacuum pump 100 is stopped. Each time the oil-sealed rotary vacuum pump 100 is to be stopped, the air introduction valve (V2) is opened to return the rotor chamber of the oil-sealed rotary vacuum pump 100 to atmospheric pressure, thereby preventing backflow of a lubricating oil.
To prevent the oil from flowing backward to the upstream side, a pump is available which has a shut-off valve provided in a region ranging from an inlet for air to a rotor chamber of the pump (see, for example, Patent Document 1).
With such a pump, a pump body is driven to introduce air from an inlet and discharge air from an outlet. A hydraulic pump which is interlocked with the pump body is also provided, and a lubricating oil is supplied by the hydraulic pump to the pump body. A shut-off valve which opens and closes the inlet is disposed in a flow path from the inlet to the pump body. The shut-off valve is adapted to be urged in a direction in which it closes the inlet, and to open the inlet under the pressure of the lubricating oil pressure-fed by the hydraulic pump.
With the pump of the above configuration, when the pump body is operated, the hydraulic pump is also actuated. Upon this actuation, the shut-off valve is brought to an open state under the pressure of the lubricating oil. Air is introduced into the pump body from the inlet, whereby the vacuum vessel can be vacuumized. When the pump body is stopped, the hydraulic pump is also stopped, so that the pressure of the lubricating oil lowers. Thus, the shut-off valve closes the inlet under its urging force, and can prevent backflow of the lubricating oil.